(a) Field of the Invention
The present invention relates to a method of treating an aqueous suspension of kaolin to separate impurities therefrom, e.g. to improve the brightness and other properties of the kaolin. The method involves the process of selective flocculation.
Kaolin or kaolinitic clay is a mineral clay containing the particulate mineral kaolinite as its principal constituent. Such clays were formed in geological times by the weathering of the feldspar component of granite. Primary kaolin clays are those which are found in deposits at the site at which they were formed, and are generally present in a matrix of undercomposed granite which must be separated from the clay during the refining process for the clay. For example, kaolin clays mainly of the primary type are obtained from deposits in South West England, France, Germany, Spain and the Czech Republic. Secondary kaolin clays, which are alternatively known as sedimentary kaolin clays, are those which were flushed out in geological times from the granite matrix in which they were formed, and were deposited in an area remote from their site of formation, generally in a basin formed in the surrounding strata. For example kaolin clays obtained from deposits in Georgia, South Carolina and Alabama, USA are generally of the sedimentary (secondary) type, as are kaolin clays obtained from deposits in Brazil.
Kaolin clay is refined and used as an ingredient, pigment or filler material in a variety of application compositions, especially for filling and coating of paper, paper board and like products. Kaolin is a white mineral and is often used in such application compositions to impart, amongst other things, whiteness and brightness. However, one or more desirable properties of the kaolin may be adversely affected by the presence of impurities. Kaolin clays are generally found in association with impurities which are often present in relatively small proportions. The composition and nature of the impurities can vary considerably, depending on the geographical region from which the kaolin clay is obtained. The impurities present can significantly affect the properties of the kaolin clay.
The present invention is concerned with treating kaolin clays containing separable impurities therein, especially impurities which comprise particles which are finer than 1 μm in size, to reduce the amount of such impurities which are present.
A particular impurity which where present in kaolin is often desirable to remove is titanium, e.g. present as titania. This impurity is colored and its presence adversely affects the whiteness and brightness of the kaolin. The titania often contains at least a small percentage of associated iron oxide which either stains the surface of the titania crystals or acts as a substituent in the titania lattice. It is the colored iron oxide associated with but not easily separable from the titania which principally causes the unwanted whiteness and brightness reduction. Titania impurity is found mainly in sedimentary kaolins, e.g. from South East USA, and it is often desirable to remove this impurity from such kaolins.
Titania itself is potentially useful as an opacifying agent. Thus, it would be potentially advantageous to concentrate and collect the titania impurity while simultaneously removing it from the kaolin.
Other impurities, such as quartz, mica, phosphates, fine clay impurities such as certain smectite clay constituents and various other species, e.g. compounds containing transition elements such as iron, may also be present and may be undesirable in many kaolin product applications. In general, such impurities may be found in either primary or sedimentary kaolin clays. However, the nature and amount of the impurity types present will vary between clay types.
(b) Prior Art
Improving the properties of kaolin clays, especially the whiteness and brightness of kaolin clays by the separation therefrom of separable impurities especially titania and iron oxide associated with the titania, has been a major problem facing the kaolin industry in the prior art. Many attempts have been made to solve this problem but none has been entirely satisfactory.
Certain physical beneficiation processes such as magnetic separation (e.g. as described in WO9850161A) and particle size classification, e.g. by centrifuging (e.g. as described in U.S. Pat. No. 4,018,673), have been applied. These are effective for separation of impurities which are not fine, e.g. having a particle size substantially greater than 1 μm, but are not effective for separating fine impurities, e.g. having a particle size less than 1 μm, from kaolin clays.
Various chemical beneficiation methods have been proposed in the prior art for employment to separate fine impurities, especially discoloring titania impurities, from kaolin clays to improve their properties such as their brightness. For example, for this purpose flotation has been described in U.S. Pat. No. 3,655,038, froth flotation has been described in U.S. Pat. No. 4,472,271 and EP-A-591406 and leaching has been described in U.S. Pat. No. 4,650,521. Substantial industrial use has been made of froth flotation. However, this process can be expensive to operate and may not be adequately effective when applied to kaolin clays containing a significant amount of fine particles, e.g. having a size less than 1 μm.
Selective flocculation has been described in the prior art for separation of impurities such as titania from kaolin, e.g. as described in U.S. Pat. No. 3,701,417, WO98/57888, U.S. Pat. Nos. 5,685,900, 5,535,890 and 4,227,920. In U.S. Pat. No. 3,701,417 and WO98/57888 the process described is of a kind operated under conditions such that the titania and other impurities are separated in a flocculated layer and the product is recovered from a deflocculated layer. In the other references the titania and other impurities are separated in a deflocculated layer.
Separating the titania in a flocculated lower layer or so called underflow layer is not fundamentally an efficient process because a significant amount of kaolinite, e.g. 20% or more, typically 30% to 45% by weight, of the feed material becomes entrained with the impurity, e.g. titania. Separating the impurity in a deflocculated layer is much more efficient, e.g. the amount of kaolinite present in the impurity layer can be small, e.g. at most a few percent by weight. A selective flocculation process of this type is described in U.S. Pat. No. 6,615,987. The process described in U.S. Pat. No. 6,615,987 results in a waste stream which is somewhat enriched in titania. However, it would be beneficial to further enrich the titania content of the waste stream, both to reduce the amount of water used in the selective flocculation process and to increase the potential for recovery of a high titania product from the waste stream.